Electrochromic materials exhibit color changes or changes in optical density as a result of electrochemical reduction and/or oxidation reactions. An electrochromic material can either be present as a solid, or exist as molecular, neutral or ionic species in an electrolyte solution. These materials have been used for the creation of electrochromic cells, where the passage of electric charge causes color changes in the materials. Electrochromic cells are used in electrochromic devices of different kinds, and two principal categories of these devices can be distinguished. The two categories differ from each other mainly in the arrangement of the elements of the electrochromic cell.
The first category of electrochromic devices utilizes a sandwich construction, and is used in applications such as automobile windows, building windows, sunglasses, large billboards, mirrors with variable reflectance, sunroofs etc. In this type of electrochromic device, continuous layers of electrochromic material and electrolyte (as well as other layers of e.g. ion reservoir material) are confined between two electrodes that completely cover the layers of electrochromic material and electrolyte. For the, electrochromic device to be of use, at least one of said electrodes has to be transparent to let light through the device. This requirement is met in the prior art through the use of electrode materials such as indium-doped tin oxide (ITO), tin dioxide or fluorine-doped tin dioxide. The electrochromic materials used in these applications vary, but are often based on heavy metal oxides such as WO3 or conducting polymers such as polyaniline or polypyrrole. The conducting, electrochromic polymer poly-(3,4-ethylendioxythiophene) (PEDOT) has attracted much study, and sandwich devices incorporating this polymer have been realized.
The second category of electrochromic devices aim at providing an electrically updateable display for realization on a flexible support. U.S. Pat. No. 6,587,250, describes such a display, comprising an electron conducting material, an electrochromic material, two electrodes and a solidified electrolyte. This display allows the electrochromic material to be addressed via the electrolyte, so that the electrode architecture is not limited by the requirement that the electrodes of the voltage supply should be in direct electrical contact with the electrochromic material for electrochromic effects to occur. Such displays have the advantages of being cheap to manufacture, environmentally friendly, and possible to manufacture using conventional manufacturing processes such as printing techniques.
Matrix addressing of electrochromic pixel devices are known in the art, and can be used to control the switching of two displays, which are spatially separated from each other, such that the first display changes color at a first instance in time, and the second display changes color at a second instance in time, wherein the two instances are separated by a time difference sufficiently large for a viewer to perceiving a delay between the first and the second color change.